Microwave filter



Aug. 22, 1961 Filed June 13, 1958 W. L. WHIRRY MICROWAVE FILTER 2Sheets-Sheet 1 Pan 1 fry/z. V

M472! 4. M/(XX mar/M44 United States Patent 2,997,673 MICROWAVE FILTERWalter L. Whirry, Inglewood, Califi, assignor to Hughes AircraftCompany, Culver City, Calif, a corporation of Delaware Filed June 13,1958, Ser. No. 741,941 2 Claims. (Cl. 333-73) The present inventionrelates to a microwave filter, and more particularly, to a variablebandwith microwave ter.

For the most part, microwave filters are narrow band devices, andefforts to increase the bandwidth, as well as render them variable, haveresulted in disadvantages. For instance, one type of broadbanding hasbeen carried out by using a plurality of resonant cavities with switchesto couple the cavities in certain grouping arrangements, and so providedifferent bandwidths to the system. Another system involves the use ofmovable irises in a waveguide to make up a variable band-pass cavity. Afurther system requires a resonant cavity having adjustable screwsinserted through the wall which results in energy being coupled betweentwo modes supported within the cavity, and thereby present a variablebandwidth dependent upon the degree of coupling between the two modes.

The foregoing examples of known variable bandwidth microwave filterseach incorporate inherent disadvantages. The first mentioned system isbulky and substantially slow acting, while the latter two systems havemoving parts requiring external mechanical actuators. Also, the lattersystems are slow acting and have to be assembled so'that the movingparts make good electrical contact with the wall of the associatedconductive elements for proper operation.

In brief, the variable bandwidth microwave filter of the presentinvention comprises a single resonant cavity having an element offerromagnetic material inserted therein and single input and outputwaveguides suitably coupled thereto. The two waveguides are coupled tothe sidewall of the cavity thereby permitting suitable temperaturecompensation at the ends. Excitation of the cavity is in a mode suchthat the magnetic field is linearly polarized at the wall of the cavity.Under such a mode, the ferromagnetic element or rod couples energy intoa second similar mode which is orthogonal and ninety degrees apart intime phase. This results in a circular rotating radio-frequency magneticfield coaxially within the cavity. The output waveguide is coupledninety de grees about the circumference of the cavity with respect tothe input guide. Now, by controlling the magnetization of the rod, theamount of coupling between the two modes is controlled with the resultthat the bandwidth is variable.

I It is therefore an object of the present invention to provide a newand improved microwave filter having a variable bandwidth.

Another object of the invention is to provide a microwave filter havinga variable bandwidth between a single input and a single output withoutmoving parts.

A further object of the invention is to provide a microwave -filterwherein the bandwidth is variable in accordance with the degree ofmagnetization of a ferromagnetic element.

' A still further object of the invention is to provide a simple,compact and light weight microwave filter of variable bandwidth.

Other objects and advantages of the present invention will be apparentfrom the following description and claims considered together with theaccompanying drawings, in which: 1

FIG. 1 is a perspective view of one embodiment of the variable bandwidthmicrowave filter of the present vention;

FIG. 2 is a plan view of the filter of FIG. 1 showing a mode ofexcitation;

FIG. 3 is a series of bandwidth characteristics of the filter forvarious degrees of magnetization of the ferromagnetic rod of FIG. 1; I

FIG. 4 is a plan view of the filter of FIG. 1 showing a second mode ofexcitation; and

FIG. 5 is a two-cavity embodiment of the filter of FIG. 1.

Referring to FIG. 1 in detail, there isillustrated a microwave filteraccording to the present invention having a rectangular waveguide 11with an input port 12 for coupling to a source of microwave energy (notshown) and exciting a cylindrical resonant cavity 13 at the resonantfrequency thereof through a suitably disposed sidewall coupling aperture14. A rectangular waveguide 16 is coupled to cavity 13 by anothersuitably disposed sidewall aperture 17 that is displaced ninety degreeswith respect to input aperture 14 and serves as an output coupling atport 18.

For operation as a microwave filter a rod 21 of ferromagnetic material,such as a ferrite, is suitably mounted within cavity .13 on the axisthereof and is magnetized by a movable, permanent magnet or, as shown inFIG. 1, an electromagnet 23. The electromagnet 23 is suitably energizedby a conventional direct current power supply 26 with a variable elementincluded in the circuit, such as potentiometer 27. Where substantiallyhigh values of static magnetic field are desired, the rod 21 may be sub'stantially small and positioned at one end of cavity 13, but wheresmaller values of field are desired, a longer rod is used, as shown inFIG. 1.

An example of modes of excitation during operation is shown in FIG. 2and reference is made to such figure for consideration with thefollowing description. Input energy at port 12 is propagated throughwaveguide 11 in a transverse electric mode, such as a TE -mode, having aplane polarized magnetic field 31. When the frequency of input energy atport 12 is far oif the value of the resonant frequency of cavity 13, ahigh impedance is presented at aperture 14 by the cavity and the energyis reflected. As the frequency of such input energy approaches the valueof the resonant frequency of cavity 13, an increasing amount of energyis coupled into the cavity to excite a mode, which for the particularcavity shown in FIGS. 1 and 2 is a transverse magnetic mode 36, such asthe TM type, and is maximized about the value of resonant frequency.Because of the ninety degree displacement between the two apertures 14and 17, none of the energy of the cavity mode 36 is coupled throughaperture 17 for propagation by waveguide 16 to output port 18.

The foregoing has been set forth with respect to zero magnetization ofthe ferromagnetic material of rod 21. Now, when a substantially lowvalue of static magneti: zation, indicated as H in the drawings, of rod21 is established by a suitable setting of potentiometer 27' acrosspower supply 26, a portion of the energy of the magnetic field 36 iscoupled by the rod into a second: similar magnetic field mode 38 whichis orthogonal and ninety degrees apart in time phase. Since the secondfield 38 is orthogonal to the first field 36, and there is a ninetydegree spacing between apertures 14 and 17, such latter aperture '17 iselectromagnetically coupled to the second field and an output isproduced at port 18.

As the frequency of the input energy is increased from a value below theresonant frequency to a value above, the output at port 18 increases toa maximum at the resonant frequency and then decreases. With an.increase in magnetization of rod 21, the amount ofpower coupled into thesecond field 38 and thus to the output pout 18 is increased, as is thespread of frequencies that excite cavity 13. Now, it is well known thatwhen an ax alrnagnetic field is applied to ferromagnetic mater aldisposed in a cavity, such combination of magnetlzed rod 21 and cavity13 results in a splitting of the resonant frequencies for clockwise andcounter-clockwise directions of rotation of the radio-frequencymagnetic. field in the cavity. The difference between the two resonantfrequencies increases as the degree of magnetization is increased.

Also, in connection with the foregoing, it is readily apparent that bothapertures 14 and i7 couple to either d1rection of rotation of theradio-frequency magnetic field. The result of such characteristics ofthe described device is to spread the lower and upper values offrequency in which cavity 13 is excited. Eventually, as the degree ofmagnetization of rod 21 is increased, the difference between the twovalues of resonant frequency becomes so great that the separation isnoted at output port 18 by two peaks of power separated by lower values.

The above described operation may be more readily understood byreference to FIG. 3 wherein a series of frequency responsecharacteristics are illustrated by plotting frequency versus poweroutput at various values of mangetization of rod 21. Thus, for zeromagnetization of rod 21, FIG. 3A shows that there is zero output at port18 for all frequencies. By applying a low value of magnetization to rod21, excitation of cavity 13 commences and as shown in FIG. 3B, at avalue of frequency quite close to the resonant frequency i of cavity 13,without the ferromagnetic material, increases to a substantially lowvalue of power and then decreases in the same manner. FIGS. 3C and 3Dillustrate frequency response curves, similar to that of FIG. 2B, forincreasing values of magnetization, and. it is seen that the frequencyspread of excitation of cavity 13 is increased as well as the amplitudeof the power output.

For a still greater increase in the magnetic field energization of rod21 to a value where the two resonant frequencies are considerablydifferent, the aforementioned peaked power output at port 18 is obtainedas shown in FIG. 3E. Thus, as indicated in FIGS. 3BE by the doublearrows 4-1 to 44 and labeled BW to 8W respectively, as the staticmagnetic field of rod 21 is increased, the bandwidth as conventionallymeasured, also increases thereby readily providing a variable bandwitdhcharacteristic to the microwave filter of the present invention.

The mode of excitation of cavity 13 is not limited to that illustratedin FIG. 2 and may readily be accomplished as shown in FIG. 4 foroperation in a manner similar to that set forth in the precedingparagraphs. In this latter drawing, the same reference numerals indicatelike elements with respect to FIG. 1. Thus energy propagated withinwaveguide 11 in a transverse electric mode 51, such as the TE -mode, iscoupled to cavity 13 in a transverse electric mode, as for example, theTB, mode which has a magnetic field 52 parallel to the longitudinaldimension of the cavity wall. With a transverse electric mode, a disc 56of ferromagnetic material is mounted at one end of cavity 13 in place ofthe rod 21 of FIGS. 1 and 2, to prevent disturbance of the electricfield of the cavity mode. By establishing a static magnetic fieldaxially through the cavity 13, the ferromagnetic disc 56 then couplesenergy into a second magnetic field 57 having the orthogonal and ninetydegree time phase difference as previously set forth with respect to thefilter of FIGS. 1 and 2. The operation of the cavity, together 'with thewaveguides ill and 16 with the foregoing modes of excitation, is thesame as described above to provide a variable bandwidth microwavefilter. Also; by applyingthe same principles square cavities may be usedin place of the cylindrical resonant cavities illustrated in thedrawing.

While the bandwidth of the filters described so far have been measuredto be variable by a factor of about 2 for conditions varied as betweenthose illustrated by FIGS. 3B and 3D and of about 3 as between FIGS. 3Band 3E, where a dip of substantially 1 decibel is permitted betweenpeaks, there are occasions where a greater factor of bandwidth variationis desired. To obtain such greater factor of bandwidth variation, tworesonant cavities 61 and 62 are cascaded by providing a central couplingaperture 63 through'adjacent ends of the two cavities as shown in FIG.5.

Each of the two cavities 61 and 62 respectively contains a ferromagneticrod or disc 66 and 67 which are magnetized as indicated by field arrow68 to establish a greater difference between resonant frequencies in onecavity than in the other. By suitable proportionment of relative sizesof the ferromagnetic rods 66 and 67, a single static magnetic fieldproducing device may be utilized to control operation of the cavities.Input energy is coupled to one cavity 61 by a waveguide 71 and sidewallcoupling aperture 72. while the output is coupled from the other cavity62 by a sidewall coupling aperture 73 and waveguide 74-.

With the foregoing structure and relationships, cavi ties 61 and 62.support, for example, TM -modes and aperture 63 couples to thecircularly polarized magnetic field of input cavity 61 to excite asimilar mode in output cavity 62. Sidewall coupling aperture 73 isninety degrees from the input coupling aperture 72 in the manner ofFIGS. 1 and 2 and operation is similar to that described for suchfigures with a resultant greater bandwidth variation.

Also, the two TM -mode cavities of FIG. 5 may be utilized in a differentmanner to achieve similar variable broadband characteristics. With suchcavity modes, the coupling aperture 63 between cavities 61 and 62 iscentered near the wall ninety degrees from the input waveguide aperture72 to electromagnetically couple to a single mode of the input cavity61. A single mode is then excited in the second cavity 62 and theferromagnetic material 67 couples a portion of the energy into a secondsimilar, but orthogonal mode. This, then, permits the output aperture 73and waveguide 74 to be positioned in linear alignment with respect toinput waveguide 71 and aperture 72. Operation is then substantially thesame as set forth in the preceding paragraphs.

A simple, compact and lightweight microwave filter has been described,which requires no moving parts to provide a variable frequency bandwidthat the output thereof. Also, changes in the bandwidth of the output maybe rapidly accomplished by a simple electrical adjustment of thestrength of the static magnetic field applied to the ferromagneticmaterial of the filter.

While the salient features of the present invention have been describedin detail with respect to a particular embodiment, it will be readilyapparent that numerous modifications may be made within the spirit andscope of the invention, and it is therefore not desired to limit theinvention to the exact details shown except insofar as they may bedefined in the following claims.

What is claimed is:

l. A variable bandwidth microwave filter comprising a first resonantcavity having ferromagnetic material mounted on a center line thereof,variable means disposed adjacent said cavity for establishing a magneticfield along said center line, input means coupled to said cavity forexciting a first mode having a magnetic field component linearlypolarized at the walls of said cavity whereby a second similar mode isexcited in said cavity through said ferromagnetic material, said secondmode being orthogonal and ninety degrees apart in time phase withrespect to said first mode, a second resonant cavity mounted end-to-endalong said center line with respect to said first cavity and having adifierent amount of ferromagnetic material subject to said magneticfield, coupling means extending between circularly polarized magneticfield components of said first cavity to similarly excite said secondcavity, and output means electromagnetically coupled to a magnetic fieldcomponent parallel to the wall of said second cavity.

2. A variable bandwidth microwave filter comprising a first cylindricalcavity having a ferromagnetic element axially mounted therein, meansdisposed adjacent said first cavity for establishing an axial staticmagnetic field through said element, a first rectangular waveguideterminated at an input sidewall aperture of said first cavity and havingthe broadwalls thereof disposed transverse to the axis of said firstcavity for exciting in such cavity a first transverse magnetic modehaving a linearly polarized magnetic field component at the ends of saidfirst cavity, a portion of said first mode being coupled by said elementto a second similar mode that is orthogonal and ninety degrees apart intime phase with respect to said first mode, a second cylindrical cavitymounted end-toend with respect to said first cavity and having aferromagnetic element of different size axially mounted therein subjectto said axial static magnetic field, a coupling aperture communicatingbetween said first and second cavities at the adjacent ends thereof toelectromagnetical- 1y couple to at least one of said modes of said firstcavity to excite two modes that are orthogonal and ninety degrees apartin time phase with magnetic field components parallel to the cylindricalwall in said second cavity, a second rectangular waveguide terminated atan output aperture of said second cavity and having the broadwallsthereof disposed transverse to the axis of said cavities for excitationby the linearly polarized magnetic field of one of said two modes insaid second cavity, and means for varying said static magnetic fieldmeans to control the degree of coupling between modes in each of saidcavities and thereby the frequency bandwidth of the output.

References Cited in the file of this patent UNITED STATES PATENTS2,632,808 Lawson Mar. 24, 1953 2,759,099 Olive Aug. 14, 1956 2,810,890Klopfenstein Oct. 22, 1957 2,825,765 Marie Mar. 4, 1958 2,944,232Beljers et al. July 5, 1960 FOREIGN PATENTS 1,099,724 France Mar. 23,1955 OTHER REFERENCES Article I, Ferrite-Tunable Microwave Cavities, byC. Nelson. Proceedings of the IRE, October 1956, pages 1449-1455 reliedupon.

